Rotary drag bits have been used for subterranean drilling for many decades, and various sizes, shapes and patterns of natural and synthetic diamonds have been used on drag bit crowns as cutting elements. In many formations, a drag bit can provide an improved rate of penetration (ROP) of the drill bit during drilling over the ROP of a tri-cone drill bit.
Over the past few decades, rotary drag bit performance has been improved with the use of a polycrystalline diamond compact (PDC) cutting element or cutter, comprised of a planar diamond cutting element or table formed onto a tungsten carbide substrate under high temperature and high pressure conditions. The PDC cutters are formed into a myriad of shapes including, circular, semicircular or tombstone, which are the most commonly used configurations. Typically, the PDC diamond tables are formed so the edges of the table are coplanar with the supporting tungsten carbide substrate. Bits carrying PDC cutters, which for example, may be brazed into pockets in the bit face, pockets in blades extending from the face, or mounted to studs inserted into the bit body, have proven very effective in achieving a high rate of penetration (ROP) in drilling subterranean formations exhibiting low to medium compressive strengths. The PDC cutters have provided drill bit designers with a wide variety of improved cutter deployments and orientations, crown configurations, nozzle placements and other design alternatives previously not possible with the use of small natural diamond or synthetic diamond cutters. While the PDC cutting element improves drill bit efficiency in drilling many subterranean formations, the PDC cutting element is nonetheless prone to wear when exposed to certain drilling conditions, resulting in a shortened life of a rotary drag bit.
PDC cutters comprise combining synthetic diamond grains with a suitable solvent catalyst material to form a mixture. The mixture is subjected to processing conditions of extremely high pressure/high temperature (HPHT) where the solvent catalyst material promotes desired inter-crystalline diamond-to-diamond bonding between the grains, thereby forming a PDC structure. The resulting PDC structure has enhanced properties of wear resistance and hardness. PDC materials are useful in aggressive wear and cutting applications where high levels of wear resistance and hardness are desired. The cutting elements used in such earth-boring tools often include polycrystalline diamond compact (often referred to as “PDC”) cutting elements, which are cutting elements that include cutting faces of a polycrystalline diamond material. Polycrystalline diamond material is material that includes inter-bonded grains or crystals of diamond material. In other words, polycrystalline diamond material includes direct, inter-granular bonds between the grains or crystals of diamond material. The terms “grain” and “crystal” are used synonymously and interchangeably herein.
PDC cutters typically include a metallic substrate material that is joined to a layer or body of the PDC material during the same HPHT process that is used to form the PDC body. The metallic substrate facilitates attachment of the PDC cutter to a drill bit. Techniques are used to improve the wear resistance of the PDC cutter which is known to suffer thermal degradation at a temperature starting at about 400° C. and extending to 1200° C. Conventional PDC cutters are known to have poor thermal stability when exposed to operating temperatures above 700° C. Some of the techniques for improving wear resistance of a PDC cutter are directed to improving the thermal stability of the PDC cutter. One technique of improving thermal stability of a PDC cutter is to leach the uppermost layer of PDC cutter to remove substantially all solvent metal catalyst material from the PDC cutter surface while retaining as much metal catalyst material in the remaining portion of the PDC cutter.
While this technique improves the thermal stability of the treated uppermost layer of a PDC cutter, such a PDC cutter tends to suffer from spalling and de-lamination during use.
Therefore, it is desirable to provide a PDC cutter having improved wear resistance properties and thermal stability which reduces or minimizes spalling and de-lamination of the PDC cutter without leaching the uppermost layer of the PDC cutter to remove solvent metal catalyst material from the PDC cutter.